Obstructive sleep apnea is a common disorder affecting approximately 4 percent of the adults in the United States. The disorder is characterized by recurrent sleep induced collapse of pharyngeal airway. The investigators believe that obstructive sleep apnea is due to a primary defect in pharyngeal anatomy. During wakefulness, there is compensation for the small airway by reflex driven activation of pharyngeal dilator muscles. During sleep, this reflex mechanism is diminished or lost thus causing pharyngeal muscle activity to return to a normal level that is inadequate to sustain airway patency. In this application, the investigators propose to study normal subjects and patients with apnea to better define the mechanisms controlling pharyngeal muscle activity during wakefulness and sleep. Three groups of studies are outlined. Specific Aim 1: Neural reflexes: The investigators believe that the upper airway negative pressure reflex is critical in activating pharyngeal muscles in patients with apnea during wakefulness thus compensating for deficient anatomy. It has been previously shown that a reduction in this reflex in normal subjects occurs during stable non-REM sleep; however, this is not a time when obstructive apneas commonly occur. As a result, it is proposed that this reflex be studied (a) during sleep onset (alpha-theta transition) and (b) during stable REM sleep (when apneas are most frequent). The reflex will be tested and compared between normals and patients with sleep apnea. The major hypothesis is that this reflex will be diminished at sleep onset and further diminished or lost during REM sleep. Specific Aim 2: Chemical influences: Unlike the reflex mechanisms which are lost during sleep, the investigators hypothesize that CO2 influences on pharyngeal muscle activity are maintained during sleep thereby sustaining neuromuscular activity. Therefore, it is proposed to determine in normal individuals (a) the response of two pharyngeal dilator muscles to increases in CO2 during wakefulness and sleep and (b) the integrated relationship between CO2 influences and the negative pressure reflex on these muscles awake and asleep. Specific Aim 3: Hormonal influences: Sleep apnea is more common in men than women. It is hypothesized that female hormones (estrogen and progesterone) produce a non-state dependent augmentation of upper airway muscle activity thereby reducing airway collapsibility awake and sleep. It is proposed to determine the influence of estrogen, progesterone and testosterone on pharyngeal muscle activity and airway collapsibility during wakefulness and sleep.